1. Field of the Invention
The present invention relates to an excavator equipped with a dozer unit for performing an earth removal operation.
2. Description of the Related Art
Referring to FIG. 7, when performing an earth removal operation using a typical excavator equipped with a dozer unit 10 of related art, a blade 12 is projected in a direction indicated by an arrow a in FIG. 7. In this case, however, the dirt accumulated above the blade 12 falls backward over the top surface of the blade 12 in a direction indicated by an arrow b, and thus roughens the finished surface. To prevent this, an additional process is required, such as temporarily raising the blade 12 upward so as to sweep the upper portion of the pile of dirt in the forward direction, or backing up the excavator in order to level the ground over again. As a result, this significantly lowers the efficiency of the earth removal operation.
In order to reduce the amount of dirt falling behind the blade 12 to the greatest possible extent, the height of the blade 12 may be made sufficiently larger with respect to the width of the blade 12.
In such a structure, however, since the dozer unit 10 is disposed below an excavating attachment unit (not shown), and the excavating attachment unit may be lowered by a large degree during an excavation process, the dozer unit 10 and the excavating attachment unit could possibly interfere with each other. For this reason, the height of the blade 12 could not be made sufficiently large, thus creating a condition where the dirt can easily fall backward.
Referring to FIGS. 7 and 8, the upper part of the blade surface 15 is curved forward by a great degree in an overhanging manner. Such a shape of the blade surface 15 is an attempt to prevent the dirt from falling backward by allowing downward pressing force components Fd to be generated.
FIG. 8 illustrates the blade surface 15, which is divided into a plurality of sections NO1 to NO10 in the height direction. Each section is given an approximate line that indicates the generation status of a force component. In FIG. 8, each reference character Fu indicates an upward pushing force component that pushes dirt upward along the blade surface 15, and each reference character Fd indicates a downward pressing force component that presses the dirt downward.
The shape of the blade surface 15 is determined based on the following conditions. Specifically, in a lower surface portion 15a, which extends from the lower end to an intermediate point of the blade surface 15 and is defined by the sections NO1 to NO4, the upward pushing force components Fu are generated. On the other hand, in an upper surface portion 15b, which extends from the intermediate point to the upper end of the blade surface 15 and is defined by the sections NO5 to NO10, the downward pressing force components Fd are generated. Furthermore, a total downward pressing force ΣFd is substantially equal to a total upward pushing force ΣFu. Although the actual relationship is ΣFu>ΣFd, the ratio between the two is close to 50:50.
However, since most of the upward pushing force components Fu are countered by the downward pressing force components Fd, the actual upward pushing force remaining at the upper part of the blade surface 15 is small. This results in a state where the dirt accumulates without rolling forward and thus forms a pile of dirt having a long bottom portion L, as shown in FIG. 7. For this reason, in response to an advancing force of the blade 12, the pile of dirt may easily collapse in the backward direction.
Furthermore, according to an experiment performed by the inventors of the present invention, it has been discovered that an energy loss is actually caused when the dirt hits the blade surface 15 and that such an energy loss is one of the factors that reduces the magnitude of each upward pushing force component Fu. Consequently, according to the dozer unit 10 in which the remaining values of the upward pushing force components Fu at the upper part of the blade surface 15 are small, the dirt can easily fall backward over the blade 12, thus lowering the efficiency of the earth removal operation.